The single greatest obstacle to advancing the science of manual medicine is the inability to measure muscle and soft tissue in vivo;the effect is to obscure direct understanding of what treatments actually do. When manual treatments, and even general biomechanical manoeuvres, apply loads to the body, the soft tissues deform, transmitting force to the bone and joint structures. The strains that arise are generated by internal forces consistent with the often non-linear tissue properties. The deformations and strains carry important information on how the body is responding to treatment. These quantities can be disclosed through an appropriate interrogation by ultrasound (USN) elastography techniques and therefore, they are the focus of this investigation. This project will extend recent advances in a promising technology to evaluate soft tissue mechanics under simple and clinically relevant treatment conditions. A body of knowledge probing theoretical effects of manual treatments has begun to arise over the past three decades. Each clinical manual therapy approach assumes that a specific set of tissues is the recipient of the beneficial effects of treatment. Yet, no prior method directly monitors which tissues deform and the relative intensity of the strain that is developed in order to justify clinical assumptions or to prioritize future research. Specifically, research on how mechanical transduction of treatment into biological benefit occurs in the different tissue strata can be engaged based on evidence of which tissues are most likely being affected. Current soft-tissue USN elastography techniques, under static conditions, will be extended to quantify relative displacement and strains (active and passive) across the depth of tissue strata (e.g. subcutaneous, fascia, longissimus, intermuscular aponeuroses, and multifidus) that arise from small amplitude motions during continuous passive motion (CPM) clinical procedures and in weight bearing postures. Relative movement of the stratified layers of the back, from treatment and task-generated perturbations, will enable the elastography interrogation of the tissues. Results from this study are applicable to a number of manual treatments specifically, and to understanding the biomechanics of strain effects from the load distribution through the soft-tissues of the torso. Such evidence will empower prioritization and quantitative testing of tissue biomarkers based on evidence of relative response to treatment. Information on tissue response path as represented by displacement and strain is important to advancing the understanding of manual therapies from three perspectives: 1) informing on the mechanisms of action that may yield clinical benefit;2) detecting boundaries for and optimization of safe application;and 3) developing insights on how to improve clinical results by redirecting treatment to achieve specific effects. Project Narrative: Manual treatment offers benefit to some patients suffering from back pain but little is known about which of the many tissue layers are affected. This study will help identify which tissues may be stimulated sufficiently to be a source for the clinical effects of treatment and to prioritize future work to understand mechanisms of back pain and to improve care.